Current output (or current-mode) digital-to-analog (D/A) converters sum together currents based on a set of digital input values to create an analog output current representative of the digital input values. Such D/A converters (DACs) typically sum a number of weighted values of a base (or reference) current to create the analog output current. The number of weighted values is dependent of the desired resolution (or number of bits) of the DAC. The weights can set such that the currents being summed are equal, or have a particular relationship, such as linear, binary, or logarithmic relationship. A particular DAC design has a fixed maximum current output value and a fixed output range of current values determined by the reference current, the weighting, if any, and the resolution of the DAC design.
Use of a particular DAC design in a variety of different applications or with a number of varying load devices can lead to inefficient usage of the DAC for some of the applications or loads. For example, in an application where the desired range of an input signal is only a portion of the full range of the DAC output, only a portion of the DACs input range will be used. The portion of the DACs input range used must still satisfy the input resolution requirements of the application, which can lead to a sub-optimum design point for applications that require the full output range (and consequently the full input range) of the DAC.
Moreover, varying applications (and loads) can require varying output ranges centered at different nominal current values. With a fixed DAC design, it is necessary for these varying applications (and loads) to use different portions of the input range of the DAC to achieve the desired output range. This can lead to having to over-design the input/output range of the DAC to accommodate for the varying ranges required by applications or load environments in which the DAC will be used.
Accordingly, a current-mode D/A converter is described having variable output and offset control. According to an exemplary embodiment, a current-mode D/A converter having variable output control includes a D/A converter having a number of first control inputs and an output capable of generating a first current proportional to a number of active first control inputs. A driver is included having an input connected to the output of the D/A converter, a number of second control inputs, and an output capable of generating a second current proportional to the first current based on a number of active second control inputs.
According to another exemplary embodiment, a current-mode D/A converter is described having variable output and offset control includes a first D/A converter having a number of first control inputs and an output capable of generating a first current proportional to a number of active first control inputs. A driver includes an input connected to the output of the first D/A converter, a number of second control inputs, and an output capable of generating a second current proportional to the first current based on a number of active second control inputs. A second D/A converter includes a number of third control inputs and an output capable of generating a third current proportional to a number of active third control inputs. Offset control circuitry includes an input connected to the output of the second D/A converter, an offset control input, and an output connected to the output of the driver. The offset control circuitry is capable of summing the second and third currents at the output of the driver when the control input is active.
According to another exemplary embodiment, a method is described for converting a digital input to a current having variable output and offset values including generating a first current proportional to a number of active first control inputs, the first current having a fixed maximum output value and a fixed offset value. A second current is generated having a variable maximum output value proportional to the first current based on a number of active second control inputs. A third current is generated that is proportional to a number of active third control inputs, the third current having a fixed maximum output value. The first and third currents are added together when an offset control input is active to form an output current proportional to a number of active first control inputs and having variable output and offset values based on the active second and third control inputs, respectively.